1. Field of the Invention
This invention relates to lightweight piston design, and more specifically to improved structures for a carbon-carbon composite piston.
2. Description of the Related Art
Internal combustion reciprocating engines and compressors used for aerospace, military, and transportation applications must be lightweight and capable of operating at elevated temperatures and pressures. Current state-of-the-art of piston manufacture employs aluminum alloys and steel because pistons composed of these materials can withstand the relatively high temperatures and pressures associated with operation of an internal combustion engine or compressor. However, engine and compressor pistons manufactured of steel and/or aluminum alloys are very heavy.
Aside from weight concerns, one disadvantage of aluminum alloy pistons is an inherent, relatively high coefficient of thermal expansion, which necessitates larger clearances between an aluminum alloy piston and an engine cylinder liner, to allow for expansion of the aluminum alloy piston during engine warm-up. Even at operating temperature, a clearance has to be maintained to allow for further piston expansion in the event that the engine experiences overheating due to intermittent, heavy loading or accidental loss of coolant. In order to seal the gap between an aluminum alloy piston and engine cylinder liner, the piston must be fitted with a plurality of piston rings which effectively seal the gap.
Another drawback associated with operation of an aluminum alloy piston at operating temperatures above 500 degrees Fahrenheit (.degree.F.), is a dramatic decrease in mechanical strength. This strength loss precludes locating the uppermost compression piston ring close to the top or crown of the piston and may result in piston failure. As a result of the restriction on placing the uppermost piston compression ring close to the piston crown, a crevice between the piston crown and the uppermost piston compression ring is created. Because fuel mixture which reaches this crevice does not burn completely, the crevice contributes to atmospheric pollution and reduces fuel efficiency. Steel pistons have sufficient strength at higher temperatures but are excessively heavy.
Yet another disadvantage of aluminum alloy pistons also results from the thermal expansive properties of the alloy. As undersized aluminum alloy pistons "rock" inside the cylinder chamber, they can be noisy until such time as they have sufficiently expanded in size to better fit the cylinder chamber dimensions. Carbon-carbon composite pistons have greater dimensional stability, i.e. are less thermally expansive and therefore consistently retain their dimensions, hence producing no engine knocking during cold operation.
In a recent development, internal combustion engine and/or compressor pistons are fabricated from carbon-carbon composite materials to reduce engine weight, improve engine efficiency, reduce hydrocarbon emissions, potentially eliminate the need for piston rings, and produce a less noisy engine. These carbon-carbon composite pistons retain their strength under operating conditions which exceed 1200.degree. F., and are lighter than either steel or aluminum alloy pistons.
Carbon-carbon composite pistons, however, are fabricated by manually laying-up and molding prepregged fabrics, tapes, or felted preforms. Such processes are labor-intensive and, therefore, expensive. These carbon-carbon composite pistons also have reduced interlaminar strength and exhibit anisotropy in their mechanical properties, thermal expansion, and thermal conductivity.